( 878 ) 



tlic ratliation emitted bv tlie pliotospliere ; so it stojjs almost V.i) ^'i*^' 

 only a small IVaction oF tiiis absorbed energy leaves the ïSun in the 

 tbi'm of radiation, emitted b\' the atmosphere itself. After all, more 

 tiian half of the radiation coming from the photosphere is retained 

 by the absorbing layer, and we cannot snppose it to go back to 

 the interior without violating the second law of thermodynamics. 

 As long as it has not been shown how the solar atmosphere may 

 get rid of that immense quantity of energy continually supplied and 

 never radiated, similar considerations will remain very unsatisfactory. 

 Our problem appears to be inuch less intricate when viewed from 

 the stand-point taken by Schmidt ^), though the mathematical treat- 

 ment will not be easy. A uniformly luminous sphere surrounded 

 by a concentric, perfectly transparent refracting envelope, will offer 

 the aspect of a disk the brightness of which diminishes towards the 

 limb. This has been established approximately by Schmidt for the case 

 of a homogeneous, sharply limited envelope. It is easily understood 

 that a similar result must be obtained when assuming a transparent 

 atmosphere of gradually decreasing density and refractive power ; 

 but then, of course, the rate at which the luminosity varies on the 

 disk will depend on the law of density variation. We may proceed 

 a little farther, and accept Schmidt's hypothesis that the incandescent 

 core of the Sun is not a sphere with a sharp boundary, but a gaseous 

 body the density and radiating power of which are smoothly dimi- 

 nishing along the radius. In this way, I think, we dispose of pre- 

 mises from which it seems possible to derive an explanation of the 

 general aspect of the solar disk without involving into such serious 

 difficulties as were hithei-to encountered. 



Chemistry. — "On the nitration of ortho- and metadibromohenzene.'' 

 By Prof. A. F. Holleman. 



(Communicated in tlie meeting of January 27, 1906). 



After the disturbing influence which the halogen atoms exercise 

 on each other's directing influence in regard to the nitro-group, had 

 been noticed in the nitration of the dichlorobenzenes, it was necessary 

 to extend this research to the nitration of the dibromobenzenes so 

 as to be able to find the connection between the results with the 

 dichloro- and dibromocompounds and to compare the same with the 

 result of the nitration of the corresponding monohalogen benzenes. 



1) A. Schmidt, Pliysik. Zeitsclir. 4, 282, 341, 453, 470 ; 5, 07, 528. (1903 and 1904). 



